In normal vehicle operation, a power source such as an internal combustion engine generates power which is transmitted to vehicle wheels through a drivetrain. In some conditions, however, power flows in the opposite direction through the drivetrain. For example, when a vehicle is slowing under engine compression braking, power flows from the vehicle wheels to the internal combustion engine. The drivetrain often includes gearing that either increases the torque and decreases the speed or increases the speed and decreases the torque. When the torque direction reverses, there may be a brief period of torque interruption as shafts rotate to bring opposite faces of the gear teeth into contact with one another. This is called a lash crossing. Some energy may build up in driveline components during this interval which is released as audible noise or a torque disturbance which may annoy vehicle occupants.
The magnitude of the disturbance is strongly correlated with the rate of change of torque during the lash crossing. To mitigate the impact of a lash crossing, a powertrain controller may limit the rate of change of torque as the torque transitions from positive to negative. Typically, this is not noticeable to vehicle occupants because the negative torque associated with engine braking is far less in magnitude than the positive torque associated with normal driving. However, the magnitude of a negative torque associated with regenerative braking in a hybrid vehicle may be much more than typical engine compression braking.